Test heads are known for probing or making electrical test connections with test points on a generally planar unit under test. The unit under test may be a printed circuit board or a back plane panel and may be loaded with components (i.e., loaded board) or unloaded (i.e., bare board). Typically, test heads have an array of spring loaded probes arranged in a grid or array according to the desired test pattern on the unit under test. Mechanical, pneumatic and vacuum actuating systems are employed to bring the array of spring probes and the unit under test into electrical contact. The spring probes are in turn connected to an analyzer which analyzes the unit under test to determine such things as continuity or lack of continuity between test points and proper operation of electronic circuitry.
One type of prior art test head, disclosed in U.S. application Ser. No. 914,726 filed June 12, 1978 now U.S. Pat. No. 4,209,745, assigned to the same assignee as the present application, is a test head system having an interchangeable test head. An interchangeable test head is employed with an array of spring probes for probing a particular array of test points on a unit under test. When it is desired to test a unit having a particular array of test points, an interchangeable test head is selected having the appropriate array of spring probes.
The interchangeable test head of the prior art includes a base plate mounting an array of spring probes, a support plate for a unit under test, and a guide apparatus for guiding the support plate with a mounted unit under test toward and away from the base plate. A resilient seal extends between the support plate and the base plate and has a free edge adjacent the base plate to thereby form a vacuum chamber between the plates. Vacuum applied in the vacuum chamber draws the unit under test, and hence the mounting plate, toward the base plate until the spring probes engage and make direct electrical connection with the unit under test.
The aforementioned prior art test head system includes a low insertion force interface system to connect the spring probes to conductors going to an analyzer. The low insertion force interface system includes interface pins mounted on the interchangeable test head and an actuable interface assembly. When the interchangeable test head is mounted in the test head system, the actuable interface assembly moves interface pins located therein into electrical engagement with the interface pins in the interchangeable test head.
Such an interchangeable test head system has certain advantages over prior art devices including greater simplicity and lower cost. However, it is desired to further simplify and reduce the cost of test head systems which are yet capable of probing a wide variety of arrays of test points.
Another technique that is used in hydraulically operated test heads is to provide a spring probe for each possible test point for all possible units under test. This means that for any particular unit under test there will be unused or redundant spring probes. By way of example, in one application, 20,000 spring probes would be required in a rectangular grid spaced 0.1 inch apart. However, this technique is not practical in vacuum test heads since the force required to overcome all of the spring probes at one time would be in the order of 5,000 pounds and could not be achieved with conventional vacuum heads.
Another prior art device is disclosed in Technical Bulletin TB7005-1 dated October, 1976 by Ditmo International and titled Formed Pin Transition Board. The technical bulletin proposes to add a T-board to a basic fixture matrix so as to transfer the position of spring loaded contact probes from a 0.1 inch grid location to random off-grid locations. The basic fixture matrix includes a fixed grid of spring probes. The T-board has two separate guide plates, consisting of a lower plate and an upper plate, in a spaced apart assembly. Pins extend through holes provided in both the upper and lower guide plates. One end of the rigid pin probes a printed circuit board and the other end engages the ends of the spring loaded contact probes. The pins are free to move in the holes provided in the upper and lower plates so that the spring loaded contact probes resiliently urge the pins against the printed circuit board. The purpose of the T-board is to probe off-grid from the grid of spring loaded contact probes. Therefore some of the pins are bent along the length thereof. The conceptual drawing in the technical bulletin shows less pins than there are spring loaded contact probes.